Wireless communication between electronic devices is becoming increasingly in demand, particularly due to the growth of multimedia communication services, such as video streaming, video conferencing, packet data transfer and so on. Accordingly, wireless networks are widely deployed to support these services. Generally, these networks are capable of supporting communications for multiple users by sharing the available network resources. One example of such network is a wireless local area network (WLAN).
The Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard denotes a set of WLAN air interface standards developed by the IEEE 802.11 working group for short-range communications (e.g., tens of meters to a few hundred meters).
The introduction of IEEE 802.11ac amendment mandates high-throughput WLAN operation in the 5 GHz band, where there is relatively less interference and more channels are available compared to the 2.4 GHz band. According to the 802.11ac standard, the specification will enable multi-station WLAN throughput at high data rates, while using a wide bandwidth (up to 160 MHz), advanced MIMO (Multiple-Input Multiple-Output) technologies, and high-density modulation (up to 256 QAM (Quadrature Amplitude Modulation)).
The IEEE 802.11ac standard utilises a number of technologies that have been utilised within previous IEEE 802.11 standards and builds on these technologies, while adding a number of new techniques to ensure that the required throughput can be attained. For example, the IEEE 802.11ac standard utilises OFDM (Orthogonal Frequency Division Multiplexing) that has been successfully used in previous forms of 802.11 standards.
OFDM is a well-known technique for transmitting high bit rate digital data signals. Rather than modulate a single carrier with a single high rate data stream, the data is divided into a number of lower rate data streams each of which is transmitted on a separate subcarriers. In this way the effect of multipath fading is mitigated. In an OFDM signal the separate subcarriers are spaced so that spectrum of subcarriers overlaps. The subcarrier frequencies are chosen so that the subcarriers are mutually orthogonal, so that the separate signals modulated onto the subcarriers can be recovered at the receiver. One OFDM symbol is defined by a set of symbols, one modulated onto each subcarrier (and therefore corresponds to a plurality of data bits). The subcarriers are orthogonal if they are spaced apart in frequency by an interval of 1/T, where T is the OFDM symbol period.
Various modulation schemes and coding rates are defined by the IEEE 802.11 standards and are represented by a Modulation and Coding Scheme (MCS) index value, Table 1 below shows the relationships between variables that allow for maximum data rate.
TABLE 1An example of Modulation and Coding Schemes supported by theIEEE802.11ac standard for transmission of signals across a20 MHz channel, where Nss (Number of spatial streams) = 3Table 22-32-VHT MCSs for optional 20 MHz, NSS = 3MCSData rate (Mb/s)IndexModulationRNBPSCSNSDNSPNCBPSNDBPSNES800 ns GI400 ns GI0BPSK½152415678119.521.71QPSK½2524312156139.043.32QPSK¾2524313234158.565.0316-QAM½4524624312178.086.7416-QAM¾45246244681117.0130.0564-QAM⅔65249366241156.0173.3664-QAM¾65249367021175.5195.0764-QAM⅚65249367801195.0216.78256-QAM¾852412489361234.0260.09256-QAM⅚8524124810401260.0288.9
Table 1. An example of Modulation and Coding Schemes supported by the IEEE802.11ac standard for transmission of signals across a 20 MHz channel, where Nss (Number of spatial streams)=3
However, a number modulation and coding schemes are not supported by the IEEE 802.11ac standard for a certain combination of variables. For example, as shown in Table 2, the IEEE 802.11ac standard does not support transmission of signals in a 20 MHz channel using MCS 9, 256 QAM with coding rate ⅚, scheme, and where Nss, the number of spatial streams=7. This is because when block wise puncturing is applied after encoding, the total number of bits in an OFDM symbol must be a multiple of n, where n is the number of bits in a block of punctured encoded bits. This problem will now be illustrated by way of an example, with reference to FIG. 1.
TABLE 2An example of Modulation and Coding Schemes supported by theIEEE802.11ac standard for transmissions of signals across a20 MHz channel, where Nss = 7Table 22-36-VHT MCSs for optional 20 MHz, NSS = 7MCSData rate (Mb/s)IndexModulationRNBPSCSNSDNSPNCBPSNDBPSNES800 ns GI400 ns GI0BPSK½1524364182145.550.61QPSK½2524728364191.0101.1QPSK¾25247285461136.5151.7316-QAM½452414567281182.0202.2416-QAM¾4524145610921273.0303.3564-QAM⅔6524218414561364.0404.4664-QAM¾6524218416381409.5455.0764-QAM⅚6524218418201455.0505.68256-QAM¾8524291221842546.0607.79Not valid
In FIG. 1, each illustrated rectangle in each row represents data bits in an encoding process. The first row represents a sequence of information bits before encoding. For the sake of simplicity, in this example, the number of information bits, Ninfo=16.
In an OFDM communications system, the number of bits in an OFDM symbol is predefined, and is herein denoted as Ncbps. The person skilled in the art would appreciate that Ncbps depends on a number factors including modulation order, the number of spatial streams, and the number subcarriers in an OFDM symbol. In this example, the number of bits in an OFDM symbol, Ncbps, is defined as 20. Therefore, in this example, one OFDM symbol is sufficient to transmit 16 information bits. The number of OFDM symbols is herein denoted as Nsym.
The second row in FIG. 1 represents the encoded data after encoding (e.g. convolutional encoding) is applied to the information bits. In this example, the information bits are encoded at a code rate (CR) of ½, i.e. one redundant bit is inserted after every single bit. As shown in FIG. 1, the total number of encoded bits, Nebits=32.
In order to achieve a higher code rate, the encoded data is punctured according to a predefined puncturing pattern. In this illustrated example, block-wise puncturing is applied using a coding rate of ⅚, i.e. 4 out of every consecutive 10 encoded bits are punctured. In this case, the puncturing block size is 10.
The term “punctured block size”, denoted as Lpblk, is used herein to connote the number of bits in a block of punctured encoded bits. The term “puncturing block size”, herein denoted as Lpingblk, is used herein to refer to the number of bits in a block of encoded bits before puncturing is applied to the encoded bits. In the present specification, the punctured coding rate is denoted as Cr. In the illustrated example of FIG. 1, Lpblk=6, Lpingblk=10, and Cr=⅚.
It is noted that the total number of bits in the punctured encoded data must be a multiple of the punctured block size as well as multiple of Ncbps.
Essentially, the condition can be concisely expressed as the following equation:mod(Nsym×Ncbps,Lpblk)=  (1)where Nsym=ceil(Nebits/Cr/Ncbps).
It is noted that this condition can also be satisfied by padding tail bits to the information bit sequence so that the number of encoded punctured bits is in a multiple of Lpblk. For example, in the prior art, the problem is addressed by padding extra tail bits to the information bits such that the total number of bits in the punctured encoded data is in a multiple of the punctured block size, Lpblk. As shown in FIG. 2, the information bits are padded with 34 tail bits, so that the total number of bits (i.e. the information bits plus the tail bits) before encoding is applied is 50. Therefore, the total number of OFDM symbols required in this case is three. As shown in FIG. 2, the total number of encoded bits is 100. Similarly, block-wise puncturing is applied using a coding rate of ⅚ to reduce the total number of encoded bits. In this case, the three OFDM symbols contain a total of 60 bits, which is a multiple of the punctured block size of 6. However, padding of extra tail bits to the information bits reduces the spectral efficiency, which is undesirable.
Due to this problem, a number of MCSs in the IEEE802.11ac standard are disabled. For example, transmissions of signals in a 80 MHz channel using MCS 6, where Nss=3 and Nss=7, are disabled. This is because multiple codewords (i.e. the number of BCC encoders, Nes>1) are used for these configurations to reduce per BCC encoder/decoder throughput, and accordingly equation (1) above can be re-expressed as:mod(Nsym×Ncbps/Nes,Lpblk)=0  (2)where Nsym=floor (Nebits/Cr/Ncbps); and
Nes is the number of BCC encoders.
Thus, the essential problem underlying the disabled MCSs is to achieve the nominal coding rate when the number of encoded punctured bits (per OFDM symbol per codeword) is not a multiple of the puncturing block size. In the IEEE 802.11ac standard, it appears that coding rates of ⅚ and ¾ are disabled for certain configurations. It would be appreciated by the person skilled in the art that disabling these MCSs would result in the link adaptation being inconsistent. Furthermore, disabling of MCS 9 would cause a reduction of peak to data rate in some configurations, which is undesirable.